A molded article derived from a polypropylene based resin expanded particle has its superior chemical resistance, shock resistance, and compression strain recovery properties or the like as compared with a molded article derived from a polystyrene based resin expanded particle. Thus, this molded article is suitably used as a bumper core material for automobiles or the like or a variety of packaging materials.
As the above polypropylene based resin, from an aspect of foaming characteristics, there is employed a propylene-alpha-olefin random copolymer obtained by primarily copolymerizing alpha-olefin such as ethylene or 1-butene or the like with propylene. These are polymerized by employing a so called Ziegler-Natta catalyst which consists of titanium chloride and alkyl aluminum.
In recent years, there is provided a proposal in which polypropylene having a syndiotactic structure obtained by employing a so called metallocene based catalyst is employed for a substrate of a foamed article (JP 1992-224832 Unexamined Patent Publication (Kokai)). By this proposal, it is possible to produce a foamed article with a propylene homopolymer.
However, there has been a problem that polypropylene having a syndiotactic structure has a low melting point as compared with polypropylene having an isotatic structure, and is inferior thereto in mechanical properties.
Further, in JP1994-240041 Unexamined Patent Publication (Kokai) (Patent Document 1), there is proposed a polypropylene based resin expanded particle in which an isotactic polypropylene based resin polymerized by employing a metallocene based polymer catalyst is obtained as a base resin.
In this case, although the present invention is characterized in that the foam diameters of foam particles are comparatively uniform, the uniformity is not always sufficient, and further improvement has been desired.
Therefore, the object of the first associated invention of the present application is to provide a polypropylene based resin expanded particle capable of obtaining a molded article with its significantly uniform foam diameters, and having its superior surface appearance and mechanical properties; and a molded article thereof.